This autumn everybody’s talking about the drones and flat-screen televisions that were predicted in the 1989 film “Back to the Future II”— and the hoverboards and flying cars that haven’t yet materialized. Together those predictions tell us a lot about the peril of imagining the future. But if we engage in a brief back-to-the-future exercise involving American politics, we might better understand the present.

Let’s travel back to the 90th Congress and have a look at the Senate. This was the period 1967 to 1969, a tumultuous time at the end of the Lyndon Johnson years, full of rancor over Vietnam and gun control but also a period of unusual agreement on environmental issues, leading to the passage of national-parks legislation and the approval of landmark national scenic-rivers and mountain-hiking measures.

And it was a time of great struggles within the Democratic Party. On one side were a group of liberals, opposed to the war, committed to new social-welfare programs, profoundly troubled by racial divisions. Frank Church of Idaho, Edward M. Kennedy of Massachusetts and Robert F. Kennedy of New York, Eugene McCarthy of Minnesota and George S. McGovern of South Dakota were fomenting revolution in the old chamber.

They were deeply resented by the conservative old bulls of the party, particularly the Southern Democrats who retained outsized power through the seniority system. Compare for a moment the Senate delegation from Wisconsin with outspoken Democratic liberals Gaylord Nelson and William Proxmire, and the Senate delegation from Georgia with outspoken Democratic segregationists Richard Russell Jr. and Herman Talmadge. Now let’s also note that Vermont, the state that spawned the most liberal Democratic presidential candidates in 2004 and 2016 — Howard Dean and Bernie Sanders — was represented by two Republicans and had just elected its first Democratic governor after 50 Republican governors in a row, spanning 109 years.

The Democratic Party was being torn apart in those days, with a new breed of ideologues who struggled to veer the party away from the center. This was, to be sure, an ancient battle, fought as long ago as 1938 by Franklin Delano Roosevelt, but a half-century later the conflict flared into the open, raising questions about whether the Democrats, straining to retain their identity as the natural party of governance, could be relied upon to rule a complex, contentious country.

Now let’s consider how the parties have completely swapped places, so much so that it is possible to lift the language of the last paragraph and, with the slightest of adjustments, fit it easily and neatly into the paragraph that follows. Watch:

The Republican Party is being torn apart these days, with a new breed of ideologues struggling to veer the party away from the center. This is an ancient battle, fought as long ago as 1964 by Barry Goldwater, but a half-century later the conflict has flared into the open, raising questions about whether the Republicans, straining to retain their identity as the natural party of governance, can be relied upon to rule a complex, contentious country.

How did this happen? How did two established parties, one more than two centuries old and the other more than a century-and-a-half old, abandon their old dance partners, perform a do-si-do and then launch into, respectively, an Allemande Left and a Star Right?

One of the answers — to strain the square-dance metaphor — is that the music changed. The parties grew more ideological due to the great changes set in motion in that important period so long ago.

It was then that the backlash of Johnson’s Great Society began to form and traces of the conservative revolution began to emerge from the ashes of the Goldwater debacle. In California, for example, Gov. Ronald Reagan was creating a new brand of conservatism. At the same time, the ties that kept the Democrats together began to fray. The struggles over civil rights and the Vietnam War were moral struggles, and the combatants began to demonize their opponents.

The result was the beginning of a brass-knuckles era of politics that would in time replace the broad national consensus that,fueled in part by Cold War tensions, prevailed in American politics for about two decades and that permitted Republicans and Democrats together to pass important civil-rights legislation in 1964 and 1965 and the watershed Medicare legislation in 1965.

There is no such consensus today, and the Republicans are moving rightward at about the speed the Democrats moved leftward in the late 1960s. Indeed, their rightward movement may be even faster because of the new ideological rigidity of the parties.

The Democrats’ leftward movement five decades ago was blunted by the presence of conservatives; the Mississippi Senate delegation, for example, included Democrats John Stennis and John Eastland, both of whom signed the Southern Manifesto of 1956 that opposed the Supreme Court ruling against school desegregation and vowed to win “a reversal of this decision which is contrary to the Constitution and to prevent the use of force in its implementation.” Today, there are no liberals in the Republican Party, and only a handful of moderates, to slow the GOP’s rightward movement. Republicans Jacob V. Javits and John Sherman Cooper — one of John F. Kennedy’s favorite Senate colleagues — have no successors.

Today’s Republican presidential nomination fight is led by two outsiders, billionaire Donald J. Trump and retired neurosurgeon Ben Carson. The establishment moderates, former Gov. Jeb Bush of Florida and Gov. John Kasich of Ohio, are struggling after promising starts. Late last month Mr. Bush, a formidable fundraiser, cut staff salaries and dismissed costly consultants. One of the explanations is that the blue-collar wing of the GOP is solidly behind Mr. Trump — as much a newcomer to the party as the blue-collar wing that supports him.

This political reversal — the 2015 Republicans behaving like the 1968 Democrats — is the principal feature of our politics right now. Here are four vital questions it prompts:

Will the Republicans continue their rightward drift, the way the 1968 Democrats drifted ever leftward? Will the Republicans, like the Democrats of the 1960s, destroy their own establishment? Will the story of the second decade of the 21st century be the triumph of the right? Or will it be the counterrevolution against the new conservative tide? Stranger things have happened. They happened a half-century ago.

David Shribman, a Pulitzer Prize winner in journalism, is executive editor of the Pittsburgh Post-Gazette.